Ok, I appreciate all the input I received on diff ways to pipe from boiler to house. My new question is about heat exchangers. I was talking to my friend about piping my efm right to the oil boiler in " series" and he looked at me like I had two heads. I've looked at a ton of pictures u guys have posted and I don't recall anyone using a heat exchanger to isolate the coal water from the house water. Upon a little research I noticed that's kind of the norm for wood boiler design. It seems to be a simple idea, and I know heat exchangers are rated for some serious btu, but I was just wondering why If I'm missing something? Is it just a design choice? I was wondering how I was gonna tie in circulator control in house with the one in coal house. I have 3 circulatory in house. My friend says his circ runs nonstop in a loop. What say you?

Most OWB's are not "boilers",they are tea kettles, as in an open to the atmosphere , no pressure system, & as such can not be plumbed to a pressurized boiler system. Plumbing a coal "boiler"to a oil "boiler is perfectly fine & makes the most economical sense in that you can utilize the circulator/s already in place. You do not drink the boiler water,so the boiler water & the drinking water are isolated from each other. Yes to the continuous circulation between the 2 boilers in your set up.My personal preference for this topic would have been for it to stay with your first topic as a part of your whole project & the continuity of 1 topic vs many different topics for 1 project.But that's just my preference.

Yeah, I'd kind of ramble sometime. To many thoughts fighting their way to my typing finger, lol. I didn't know wood boilers were an open system. As far as the circulator running 24/7, if a warm day pops up and no heat is called for for hours, that circ still needs to run? I thought I would just wire it up to a thermostat, so they both come on with the call for heat. That's just my uneducated opinion.What would be the rationale for continuous operation of circ in coal shed?

You'd want the entire volume of water in both boilers at temperature all the time. If the volume in the oil boiler dropped too low, it would slam the coal boiler with a large volume of cold water and could cause problems and take a long time to heat up when a call for heat came in.

Got it, my circs run like crazy most of time anyway. And I haven't replaced one in 14 years pushing this water through my slab with this radiant. I don't know why I was concerned with running one constant. If I had to replace it every five years, so what.

You wouldn't want to park your car along the interstate overnite, jump in it in the a/m 0* temp.put your seat belt on, start the engine,put in drive & mash the gas pedal to the floor.Same concept with a continuously running circulator,it keeps everything ready for whatever demand comes it's way ,rather than the first scenario which will drastically shorten the life of engine/trans.